The reduction of aromatic and heteroaromatic nitro compounds provides an important synthetic route to the corresponding amines. However, the reaction proceeds over several intermediates and it is sometimes difficult to achieve a complete reduction. In particular the hydroxylamine intermediate often poses a problem since it is relatively stable and may accumulate in the reaction mixture. When the reduction is carried out via catalytic hydrogenation, other reducible substituents, such as halogen atoms or groups containing carbon-carbon multiple bonds, or sulfur-containing groups (e.g. thioether groups) can cause unwanted side reactions (e.g. hydrogenolysis or hydrogenation of carbon-carbon multiple bonds) or inhibition effects (“catalyst poisoning”), which result in formation of undesired byproducts, unsatisfactory yields, or the requirement of uneconomically large amounts of catalyst. It has been an objective of the present invention to provide a method for the selective reduction of aromatic and heteroaromatic nitro compounds to the corresponding amines that gives good yields even in the presence of halogen or oxygen-containing or sulfur-containing substituents without requiring drastic reaction conditions or unreasonably large amounts of catalyst.
According to the invention, this objective has been achieved by the processes and the catalysts described herein. It has been found that the performance of a supported platinum catalyst in the hydrogenation of an aromatic or heteroaromatic nitro compound to the corresponding amine can be substantially improved by modifying it with a molybdenum compound and a phosphorus compound wherein the phosphorus has an oxidation state of less than +5.